Electronic information handling or computer systems, whether large machines, microcomputers or small and simple digital processing devices, require memory for storing data and program instructions. Various memory systems have been developed over the years to address the evolving needs of information handling systems. Often, an information handling system will employ a variety of memory technologies, which may be grouped or distinguished, for the purposes herein, as mass storage devices and semiconductor memory.
Mass storage devices are generally nonvolatile and primarily used for information not frequently accessed. Mass storage devices are sometimes called "moving-surface memory" because they take the form of discs and tapes. Such mass storage devices have large capacities, great flexibility and low cost. Of course, moving-surface memories require relatively high power to operate and are prone to mechanical failures over the life of the devices. Also, such mass storage devices are referred to as "serial" or "sequential" memories, from which data is available only in the same sequence as it is originally stored. Consequently, such mass storage devices are hampered by longer access times that creates an inconvenience every time they are used.
In contrast, semiconductor memory is usually the most rapidly accessible memory and, thus, the one from which most instructions and recently-used data are stored. In a semiconductor memory, often the time required for storing and retrieving information generally is independent of the physical location of the information within the memory. Semiconductor memory typically store information in a large array of cells. A group of cells are electrically connected together by a bitline, or data line. An electrical signal is used to program a cell or cells. The electrical signal on the data line is controlled by the bitline driver circuit. Accordingly, a semiconductor memory device may include several groups of cells, each coupled together with a data line operated by a bitline driver. The electrical signal on the data line is supplied by a programming signal provided on semiconductor memory devices. The programming signal must supply a large amount of current on the data line.
Prior to shipping, a manufacturer will test its semiconductor memory devices. Among the tests performed include a bitline stress test. Generally, the bitline stress test is used to observe the effect of the electrical signal on the data line on cells not intended to be programmed. In the bitline stress test, a higher voltage than usual is applied to all of the cells in a group. During such a test, a leakage current is produced from the cells, and the total leakage current during a test can be quite substantial. Thus, in order to perform this test, a larger than usual amount of current is required. Other tests require that multiple groups of cells are programmed at the same time. These tests require a large amount of current which is itself multiplied by the number of groups programmed in parallel.
The large amount of current required for these tests can have a deleterious effect on the circuitry of the memory device. For example, in flash memory devices, a significant type of semiconductor memory device, the source of the programming signal is often on the opposite side of the chip from the bitline driver. A wide line able to supply large currents is required to travel a relatively great distance in order to provide the programming signal to the bitline driver. The amount of current carried by the line and the distance the current is required to travel effects electromigration issues that contribute to signal loss in the line and unwanted electrical interference in the memory device. Also, many of the relatively delicate circuitry elements are adversely affected by the large amounts of currents during testing of the device.